Solid-phase microextraction (SPME), a solvent-free sample preparation technique, was developed by Pawliszyn and co-workers using a fused-silica fiber externally coated with a polymeric sorbent covering a small segment of it at one of the ends. Analytes present in the sample medium were directly extracted and preconcentrated by the coated sorbent in the process of reaching an extraction equilibrium with the sample matrix. The preconcentrated analytes were then desorbed into a GC instrument for analysis.
In conventional fiber-based SPME, still there exist a number of shortcomings that need to be overcome. These include inadequate thermal and solvent stability of conventionally prepared sorbent coatings, low sample capacity, difficulties associated with the immobilization of thick coatings, susceptibility of the fiber (especially the coated end) to mechanical damage and technical difficulties associated with the hyphenation of fiber-based SPME with liquid-phase separation techniques.
Capillary microextraction (CME) (also called in-tube SPME) presents a convenient format for coupling SPME to HPLC and for automated operation of SPME-HPLC. Hyphenation of CME to HPLC is especially important for the analysis of a wide range of less volatile or thermally labile compounds that are not amenable to GC separation. In the open tubular format of CME, a sorbent coating is applied to the inner surface of a capillary. This alternative format provides an effective solution to the problem associated with the mechanical damage of sorbent coating frequently encountered in conventional fiber-based SPME where the coating is applied on the outer surface of the fiber. In this new format of SPME, a segment of wall-coated capillary GC column is commonly used for the direct extraction of organic analytes from an aqueous medium. To perform HPLC analysis, the extracted analytes are transferred to the HPLC column by desorbing them with an appropriate mobile phase.
Capillary microextraction has great prospects in liquid-phase trace analysis. However, to achieve its full analytical potential, the technology needs further improvements in a number of areas. First, segments of GC columns that are commonly used for sample preconcentration have thin coatings that limit the sorption capacity, and hence, the extraction sensitivity of in-tube SPME. Second, the sorbent coatings in such microextraction capillaries usually are not chemically bonded to capillary inner walls, which limits their thermal and solvent stabilities. Third, conventionally prepared GC coatings that are used in in-tube SPME capillaries inherently possess poor pH stability. This places serious limitations on the range of applications amenable to CME-HPLC analysis. Low pH stability of in-tube SPME coatings practically excludes the applicability of the technique to high-pH samples or analytes that require high-pH solvent systems for desorption from the microextraction capillary. Therefore, development of methodologies for the creation of high pH- and solvent-resistant sorbent coatings is an important area in the future development of in-tube SPME, which is expected to play a major role in effective hyphenation of this sample preconcentration technique with liquid-phase separation techniques that commonly use organo-aqueous mobile phases with a wide range of pH conditions.
Sol-gel chemistry has been recently applied to solid-phase microextraction (SPME) and capillary microextraction (CME) to create silica-based hybrid organic-inorganic coatings. The sol-gel technique provided chemically bonded coatings on the inner surface of fused-silica capillaries, and easily solved the coating stability problems described above.
Although sol-gel technique helped overcome some significant shortcomings of SPME or in-tube SPME techniques by providing an effective means of chemical immobilization for sorbent coatings, an important problem inherent in silica-based material systems (commonly used in SPME or CME) still remains to be solved: silica-based materials possess a narrow window of pH stability. In the context of SPME, it pertains to the stability of silica-based fibers and coatings. The development of alternative materials possessing superior pH stability and better mechanical strength should provide SPME with additional ruggedness, and versatility.
Recently, titania has attracted interest in separation science due to its superior pH stability and mechanical strength compared with silica. Several studies have been conducted on the application of titania in chromatographic separations. Tani et al. reported the preparation of titania-based packing materials for HPLC by sol-gel method, and investigated their properties. Tsai et al. prepared silica capillaries coated with titania or alumina for capillary electrophoresis (CE) separation of proteins. Fujimoto used a thermal decomposition technique to create titania coatings on the inner surface of fused-silica capillaries for capillary zone electrophoresis (CZE) and capillary electrochromatography (CEC) applications. The titania-coated capillaries were found to possess a bi-directional electroosmotic flow (EOF) and low solubility in aqueous solutions within a pH range of 3-12. Pesek et al. reported the surface derivatization of titania with triethoxysilane to prepare titania-based stationary phases via silanization/hydrosilylation. Some other groups reported preparations of silica-coated titania monolayers for faster and more efficient coating, which is important for further preparation of nanocomposites.
We disclose the preparation of sol-gel TiO2-PDMS coated capillaries and show the possibility of on-line CME-HPLC operation using sol-gel TiO2-PDMS microextraction capillaries to provide a significant improvement in pH stability and extraction sensitivity.